Setting up home-manager with nix-direnv

Goal

We’ve set up nix and home-manager with flake from our the previous post. In this post, we are going to further customize our dotfiles, and then integrate nix-direnv using home-manager so that we can start having reproducible dev environment per directory.

For those of you that are impatient, a full snippets can be found at

• https://github.com/rexk/home-manager-direnv-example/tree/nodejs-example
• https://github.com/rexk/dotfiles/tree/nix-flake-install-stage-02

Before getting started

Before getting started, let’s add a convenient shell script so that we can interact with our dotfiles bare repo easier.

Add following script at: ~/.config/bin/dit

#!/usr/bin/env sh

# ------------------------------------------------
# dit - short hand for git for dotfiles
# ------------------------------------------------
# As we control dotfiles as git's bare repo, 
# it is cumbersome to type out whole work-tree
# git command to interact with our dotfiles folder
#
# dit command gives us easy access to control out dotilfes
# without deviating too much from our `git` command flow


CONFIG_DIR=${XDG_CONFIG_HOME:=$HOME/.config}
git --git-dir=$HOME/.dotfiles.git --work-tree=$CONFIG_DIR "$@"

direnv and nix-direnv

direnv let me control directory level dev environment setup with .envrc file. But I would like to use this with nix’s flake. Luckily there is a community project laready exists with nix and direnv integration, nix-direnv.

With home-manager, it is really simple to integrate nix-direnv with my current system.

@@ -5,5 +5,7 @@ in
   home.username = user.name;
   home.homeDirectory = user.homeDir;
   home.stateVersion = "22.11";
+
+  programs.direnv.enable = true;
 }

After the file is updated, now we need to run our hms (home-manager switch) command.

We can verify our installation by running direnv from the terminal.

direnv --version
2.32.1

Now that, we have dirnev installed, it is time for us to test out our direnv setup.

I would like to set up a nodejs project using express NodeJs version with 18.x LTS.

1. Create a project directory
2. Add flake.nix
3. Update flake.nix to pull nodejs 18.x
4. Add .envrc
5. Execute direnv allow

mkdir -p nodejs-18-example
cd nodejs-18-example

Then, we are going to add following flake.nix at the root of nodejs-18-example

{
  description = "nodejs-18-example-flake";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-22.11";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { nixpkgs, flake-utils, ... }:
    let
      utils = flake-utils;
    in
    utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
      in
      {
        formatter = pkgs.nixpkgs-fmt;

        devShell = pkgs.mkShell {
          nativeBuildInputs = with pkgs; [
            nodejs-18_x
          ];
        };
      }
    );
}

Before we proceed further, flake.nix file must be commited into git.

git add flake.nix
git commit -am "Add flake.nix"

Once flake.nix file is committed, we are going to add .envrc file with the following content:

use flake

Once both files are successfully added, we are going to see following error messages:

direnv: error /home/rexk/rexk-workspace/home-manager-direnv-example/nodejs-18-express/.envrc is blocked. Run `direnv allow` to approve its content

Run direnv allow so that direnv can do its job.

Now, nix and direnv will do its magic, and download nodejs for us.

node --version
v18.14.1

As long as flake.lock file and flake.nix file does not change, nodejs-18-express will alawys use same nodejs versions no matter who enter this project. Neat